Fire protection in a piston-cylinder unit

ABSTRACT

Piston-cylinder unit comprising a cylinder in which a work space is filled with a work medium and which is limited by a closure and is sealed by at least one seal. The closure is fixed axially by a positive engagement with the cylinder up to a temperature threshold and the closure can carry out an axial movement for relieving pressure above the temperature threshold. A cutting device cancels the sealing function of the seal during the axial movement of the closure for relieving pressure.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention is directed to a piston-cylinder unit including acylinder having an inner wall bounding a work space filled with aworking medium, and a closure fixed axially with respect to the cylinderby position engagement with the cylinder. The closure bounds the workspace and is sealed with respect to the wall by a circumferential seal,the closure being axially movable with respect to the cylinder when atemperature threshold is exceeded.

[0003] 2. Description of the Related Art

[0004] It can not be ruled out in principle that a piston-cylinder unitis subject to extreme temperature loading during operation, for example,in case of an automobile accident in which the vibration damper of themotor vehicle is exposed to fire. The work medium found in thepiston-cylinder unit undergoes a very great expansion, so that apush-out force acting on the piston rod and a closure overloads thefastening of the piston rod guide within the cylinder which is designedfor normal operation. In theory, it may happen that the piston rodtogether with the piston is pushed out of the cylinder due to thepressure level within the piston-cylinder unit.

[0005] Solutions have already been suggested for this scenario. U.S.Pat. No. 5,454,455 discloses that at least one bead provides a stopsurface for the piston rod-piston unit which moves out. This solutionrequires an adapted piston and in some cases leads to a loss of usefulstroke because the bead is formed within the travel path of the piston.

[0006] U.S. Pat. No. 5,485,987 discloses several possibilities forreducing overpressure in a gas spring. For this purpose, the gas springhas a supporting body for the piston rod seal, which supporting body isconstructed elastically in itself or is supported in a springing mannerand releases a flow-out opening within an axial displacement path of thepiston rod seal. However, there is also a solution in which the pistonrod seal is destroyed by the increased operating pressure and an outletchannel is released. The possibilities shown in this reference require asupporting body whose inherent elasticity can be tuned to a definedtemperature only with difficulty. A piston rod seal with a predeterminedbreaking point is at least appreciably more expensive than aconventional construction.

[0007] U.S. Pat. No. 4,931,688 discloses a safety fuse which releases aflow-out opening of the cylinder in case of fire. U.S. Pat. No.5,064,030 is also to be mentioned in this connection. With respect tosafety fuses, it can happen that the seal will lose its original contourand become mushy but will still continue to exert a sealing function.

SUMMARY OF THE INVENTION

[0008] It is the object of the present invention to realize a safetymechanism for extreme temperature loading in a piston-cylinder unitwhich functions in a reliable manner without significantly increasingexpenditure and without impairing the functioning of the unit itself.

[0009] According to the invention, this object is met in that a cuttingdevice cancels the sealing function of the seal during the axialmovement of the closure for relieving pressure.

[0010] A substantial advantage of the invention consists in that it isnot necessary to rely upon the melting behavior of the seal for the workmedium to flow off.

[0011] It is further provided that the cutting device is arranged insidethe cylinder. Add-on components which would take up additional externalinstallation space are not necessary.

[0012] According to an advantageous embodiment, the cutting device issupported at the cylinder. The cylinder is generally designed as apressure vessel and can absorb very high forces.

[0013] For the lowest possible expenditure on manufacturing, the cuttingdevice is constructed integral with the cylinder. There is no need foradditional structural component parts which would add to manufacturingcosts. The cutting device is formed by a tongue-like projection.

[0014] For optimum reliability of operation, the seal is arranged withina groove of the closure, and the radial groove depth is greater than theradial dimension of the cutting device. It is ensured that the cuttingdevice can engage in the functional area of the seal.

[0015] In order to prevent metallic sealing locations after the cuttingprocess, a cutting surface of the cutting device is arranged entirelyradially inside of the cylinder. In this connection, the cutting devicehas a stripping surface which removes the cut off portion of the closurefrom a core part.

[0016] In addition, the cylinder has a free space which can receive thecut off part of the closure.

[0017] The cutting device can be produced at relatively low cost in thatthe tongue-like projection of the cutting device is cut from the wall ofthe cylinder and the free cut for the projection forms the free space.

[0018] The closure has a circumferentially extending groove receivingthe tongue-like projection of the cutting device, so that the cuttingdevice does not need any additional external installation space.

[0019] Depending on the construction, the cutting surface can contact aside wall of the groove in the closure in normal operation of thepiston-cylinder unit. Accordingly, it is possible that the cuttingdevice provides additional holding or retaining forces for the closure.

[0020] Installation space in the cylinder is made use of in aparticularly favorable manner in that the circumferentially extendinggroove in the closure for the cutting device with at least one bead ofthe cylinder forms the positive engagement for the axial fastening ofthe closure within the cylinder.

[0021] It is possible to use several cutting devices which enables afaster decrease in pressure in the cylinder.

[0022] Further, when using more than one cutting device it is useful tolet these cutting devices act on the circumference of the closureasymmetrically. The cutting process causes the closure to assume anoblique state relative to the longitudinal axis of the piston-cylinderunit, so that a clamping force can be used to hold the closure in thecylinder.

[0023] Other objects and features of the present invention will becomeapparent from the following detailed description considered inconjunction with the accompanying drawings. It is to be understood,however, that the drawings are designed solely for purposes ofillustration and not as a definition of the limits of the invention, forwhich reference should be made to the appended claims. It should befurther understood that the drawings are not necessarily drawn to scaleand that, unless otherwise indicated, they are merely intended toconceptually illustrate the structures and procedures described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 shows a section from a piston-cylinder unit in the area ofthe closure; and

[0025]FIG. 2 shows a piston-cylinder unit after the temperature limithas been exceeded.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

[0026]FIG. 1 shows a section from a piston-cylinder unit 1. Thisembodiment example concerns a vibration damper constructed according tothe single-tube principle. A cylinder 3 has a work space 5 which isfilled with a work medium and which is limited at the end by a closure7. A seal 9 inside a groove 10 closes an annular gap 11 between theinner wall of the cylinder and the outer surface of the closure. Anaxially movable piston rod 13 is enclosed by a piston rod seal 15 whichprevents leakage at the inner diameter of the closure.

[0027] The closure is held in its predetermined position by a positiveengagement with the cylinder in the range of normal operatingtemperature. For this purpose, the cylinder has a quantity of beads 17which extend in circumferential direction and engage in acircumferentially extending groove 19 of the closure.

[0028] The section on the left-hand side of FIG. 1 shows a cuttingdevice 21 inside the cylinder 3. The cutting device 21 is supported atthe cylinder in that it is formed integral with a tongue-like projection23 of the cylinder 3 which is received by the circumferentiallyextending groove 19. The tongue-like projection 23 of the cutting deviceis cut from the wall of the cylinder and has a free space 25. A cuttingsurface 27 of the tongue-like projection is constructed completelyradially inside the cylinder. Further, the groove depth of the groove 10for the seal 9 is greater than the radial dimension of the cuttingdevice 21. In this embodiment example, the cutting surface 27 contacts aside wall 29 of the circumferentially extending groove 19 in normaloperation of the piston-cylinder unit and ensures an additionalretaining force in addition to the beads 17 already mentioned.

[0029] Although only one cutting device is shown, several such cuttingdevices can also be used, but should be allowed to act on thecircumference of the closure asymmetrically so as to make use of anoblique state and the consequent clamping forces between the closure andthe cylinder.

[0030] In case of fire when the piston-cylinder unit is mounted in amotor vehicle, for example, and this motor vehicle catches fire in anaccident, the temperature at the cylinder rises above a normal operatingtemperature threshold determined by the construction of thepiston-cylinder unit. The work medium is likewise heated and, in sodoing, necessarily expands. A pressure force exceeding the retainingforce of the beads 17 in the circumferentially extending groove 19 actsat the underside of the closure 7, so that the positive engagementbetween the closure and the cylinder is canceled, particularly inclosures whose material has a thermal resistance lower than that of thecylinder. After this loading point, the closure carries out an axialmovement in which the tongue-like projection 23 penetrates into the sidewall 29 of the groove by its cutting surface 27 and displaces severedparts of the closure from a core part by a strip-off surface 31.Portions 7 a severed from the volume of the closure are displaced intothe free space 25.

[0031]FIG. 2 shows the state in which a shoulder 33 between thecircumferentially extending groove 19 and the groove 10 for the seal 9is severed and the cutting surface 27 acts at the seal 9. With a furthermovement, the seal 9 is destroyed, so that the work medium can escapethrough the annular gap 11 into the free space or along the severedshoulder 33 into the surroundings.

[0032] Extensive tests have shown that no metallic sealing locations areformed between the closure and the inner wall. Further, the technicalexpenditure on manufacturing is comparatively small because an embossingcutting tool is used for the tongue-like projection 23 instead of abeading die for the beads 17.

[0033] The cutting device functions not only in connection with apositive engagement in the form of a bead, but also completelyindependent from the latter. A threaded connection or a clamped in wirering can also be used instead of the bead connection. At correspondingtemperatures, these positive engagements also tear and allow the cuttingdevice to take effect.

[0034] Thus, while there have shown and described and pointed outfundamental novel features of the invention as applied to a preferredembodiment thereof, it will be understood that various omissions andsubstitutions and changes in the form and details of the devicesillustrated, and in their operation, may be made by those skilled in theart without departing from the spirit of the invention. For example, itis expressly intended that all combinations of those elements and/ormethod steps which perform substantially the same function insubstantially the same way to achieve the same results are within thescope of the invention. Moreover, it should be recognized thatstructures and/or elements and/or method steps shown and/or described inconnection with any disclosed form or embodiment of the invention may beincorporated in any other disclosed or described or suggested form orembodiment as a general matter of design choice. It is the intention,therefore, to be limited only as indicated by the scope of the claimsappended hereto.

We claim:
 1. A piston-cylinder unit comprising: a cylinder having aninner wall bounding a work space filled with a working medium; a closurefixed axially with respect to said cylinder by positive engagement withthe cylinder, said closure bounding said work space and being sealedwith respect to said inner wall by a circumferential seal, said closurebeing axially moveable with respect to said cylinder when a temperaturethreshold is exceeded; and a cutting device which cuts said seal as saidclosure moves axially with respect to said cylinder, whereby saidworking medium can escape from said working space.
 2. A piston cylinderunit as in claim 1 wherein said cutting device is located radiallyinside said inner wall of said cylinder.
 3. A piston-cylinder unit as inclaim 2 wherein said cutting device is supported by said inner wall. 4.A piston-cylinder unit as in claim 3 wherein said cutting device isformed integrally with said cylinder.
 5. A piston-cylinder unit as inclaim 4 wherein said cutting device is formed as a tongue-likeprojection projecting radially inward.
 6. A piston-cylinder unit as inclaim 2 wherein said closure comprises a circumferential groove whichreceives said seal, said groove having a radial depth which is greaterthan the radial dimension of said cutting device.
 7. A piston-cylinderunit as in claim 2 wherein said cutting device has a cutting surfacewhich is arranged entirely radially inside of said inner wall.
 8. Apiston-cylinder unit as in claim 1 wherein said cutting device cuts offa portion of said closure as said closure moves axially with respect tosaid cylinder, said cutting device having a stripping surface whichremoves the cut off portion of the closure.
 9. A piston-cylinder unit asin claim 8 wherein said cylinder has a free space which can receive saidcut off portion of said closure.
 10. A piston-cylinder unit as in claim9 wherein said cutting device is a tongue-like projection formedintegrally with said cylinder, said tongue-like projection being formedout of said free space.
 11. A piston-cylinder unit as 5 wherein saidclosure comprises a circumferential groove which receives saidtongue-like projection, said circumferential groove having a side wall.12. A piston-cylinder unit as in claim 11 wherein said tongue-likeprojection has a cutting surface which contacts said side wall prior tomovement of said closure with respect to said cylinder.
 13. Apiston-cylinder unit as in claim 11 wherein said closure comprises atleast one bead which is received in said groove to fix said closureaxially with respect to said cylinder.
 14. A piston-cylinder unit as inclaim 1 comprising a plurality of said cutting devices.
 15. Apiston-cylinder unit as in claim 14 wherein said cutting devices aresituated asymmetrically about the axis of said cylinder.